Subversion Repositories round_robin_arbiter

//Rotate -> Priority -> Rotate

//author: dongjun_luo@hotmail.com

module round_robin_arbiter (

        rst_an,

        clk,

        req,

        grant

);

input           rst_an;

input           clk;

input   [3:0]    req;

output  [3:0]    grant;

reg     [1:0]    rotate_ptr;

reg     [3:0]    shift_req;

reg     [3:0]    shift_grant;

reg     [3:0]    grant_comb;

reg     [3:0]    grant;

// shift req to round robin the current priority

always @ (*)

begin

        case (rotate_ptr[1:0])

                2'b00: shift_req[3:0] = req[3:0];

                2'b01: shift_req[3:0] = {req[0],req[3:1]};

                2'b10: shift_req[3:0] = {req[1:0],req[3:2]};

                2'b11: shift_req[3:0] = {req[2:0],req[3]};

        endcase

end

// simple priority arbiter

always @ (*)

begin

        shift_grant[3:0] = 4'b0;

        if (shift_req[0])        shift_grant[0] = 1'b1;

        else if (shift_req[1])  shift_grant[1] = 1'b1;

        else if (shift_req[2])  shift_grant[2] = 1'b1;

        else if (shift_req[3])  shift_grant[3] = 1'b1;

end

// generate grant signal

always @ (*)

begin

        case (rotate_ptr[1:0])

                2'b00: grant_comb[3:0] = shift_grant[3:0];

                2'b01: grant_comb[3:0] = {shift_grant[2:0],shift_grant[3]};

                2'b10: grant_comb[3:0] = {shift_grant[1:0],shift_grant[3:2]};

                2'b11: grant_comb[3:0] = {shift_grant[0],shift_grant[3:1]};

        endcase

end

always @ (posedge clk or negedge rst_an)

begin

        if (!rst_an)    grant[3:0] <= 4'b0;

        else            grant[3:0] <= grant_comb[3:0] & ~grant[3:0];

end

// update the rotate pointer

// rotate pointer will move to the one after the current granted

always @ (posedge clk or negedge rst_an)

begin

        if (!rst_an)

                rotate_ptr[1:0] <= 2'b0;

        else

                case (1'b1) // synthesis parallel_case

                        grant[0]: rotate_ptr[1:0] <= 2'd1;

                        grant[1]: rotate_ptr[1:0] <= 2'd2;

                        grant[2]: rotate_ptr[1:0] <= 2'd3;

                        grant[3]: rotate_ptr[1:0] <= 2'd0;

                endcase

end

endmodule